Heat treatment of graphite fibers

ABSTRACT

A process for improving the shear strength of a graphite fiberresin matrix wherein the fibers are coated with a soluble coating compound consisting of a metal hydroxide, peroxide, halide, nitrate, nitrite, permanganate, dichromate or sulfide, and heated in a controlled atmosphere at above 400* C. prior to incorporation in the composite; such composites being useful as structural materials.

United States Patent Inventors Robert A. Cass;

Samuel Steingiser, both of Dayton, Ohio Appl. No. 41,412 Filed May 28, 1970 Patented Dec. 14, 1971 Assignee Monsanto Research Corporation St. Louis, Mo.

HEAT TREATMENT OF GRAPHITE FIBERS 6 Claims, 1 Drawing Fig.

US. Cl 117/118, 117/93, 117/47 R, 117/169 R, 117/D1G. 11,

264/D1G. 19, 23/2091 P, 23/2092 Int. Cl B44d 5/12, B44d 5/00 Field of Search 1 17/169 R,

118, 46 F2, 46 CC, 93, 47 R, D1G. 11; 23/2092, 209.1 P; 8/1 15.6; 264/D1G. 19

[56] References Cited UNITED STATES PATENTS 2,839,426 6/1958 Gerby 117/169 R X 3,366,464 H1968 Guichet et a1. 1 17/169 X Primary Examiner-Alfred L. Leavitt Assistant Examiner-Edward G. Whitby Anorneys-Morris L. Nielsen, L. Bruce Stevens, Jr. and Frank D. Shearin ABSTRACT: A process for improving the shear strength of a graphite fiber-resin matrix wherein the fibers are coated with a soluble coating compound consisting of a metal hydroxide, peroxide, halide, nitrate, nitrite, permanganate, dichromate or sulfide, and heated in a controlled atmosphere at above 400 C. prior to incorporation in the composite; such composites being useful as structural materials.

HEAT TREATMENT OF GRAPHITE FIBERS The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Air Force.

BACKGROUND OF THE INVENTION This invention relates to a process for modifying fibers and more particularly to heat treating graphite fibers that have been coated with certain metal compounds, for the purpose of improving the shear strength of a graphite fiber-resin matrix composite.

Graphite fibers having high tensile strength, e.g. over 200,000 p.s.i., and high modulus, e.g. over 20x10 p.s.i., having recently become available in the marketplace. Composites made from such graphite fibers and resin matrices, including epoxy resins, phenolics, polyesters polyimides, etc., are useful structural materials because of their high strength-to-weight ratio However, such composites have generally not attained their anticipated potential because of deficiencies in interlaminar shear strength.

U.S. Pat. No. 3,281,261, issued Oct. 25, 1966 to Lynch discloses refractory metal oxide coatings on carbonized textile fibers for improved tensile strength. US. Pat. No. 3,497,318 issued Feb. 24, 1970 to Noss discloses the action of a chemical oxidizing agent on a carbon fiber precursor prior to carbonizing. Neither reference teaches improved shear strength in a composite.

We have found that improved composites with high shear strengths can be obtained by our process without serious deterioration of other physical properties such as fiber tensile strength and without producing a pitted surface.

SUMMARY OF THE INVENTION An object of this invention is to provide a continuous process for modifying the surface of graphite fibers. Another object is to provide a process for improving the bonding between graphite fibers and a resin matrix. A further object is to provide a process for improving the shear strength of composites containing graphite fibers and a resin matrix. Still a further object is to provide a composition incorporating heattreated graphite fibers as a reinforcement.

These and other objects hereinafter defined are met by the invention wherein there is provided a process of treating a high-modulus graphite fiber to improve the bonding characteristics of said fiber to resin matrix comprising coating the fiber with about 0.0l to percent by weight of a soluble coating compound consisting of a metal hydroxide, peroxide, halide, nitrate, nitrite, permanganate, dichromate or sulfide, and heating the coated fiber in an atmosphere containing not more than ppm. of oxygen at a temperature of above 400 C. for a time sufficient to improve the shear of a graphite fiber-resin matrix composite over the shear strength of a control composite made with untreated graphite fibers.

The invention is adaptable and applicable to graphite fibers not only in the form of tow but also in yarns, tapes, felts, sheets, woven fabrics and other forms prepared from or containing graphite fibers. Although the term graphite" is used, the fibers need not be highly crystalline as determined by X- ray diffraction analysis.

The invention may be practiced as a batch operation, but for a full-scale industrial process it is preferably done continuously. Yarns provide an especially convenient form for a continuous process. By employing a continuous process, production output is increased greatly and product quality and uniformity is improved. Our product is substantially free of pitting, as shown by scanning electron micrography, and has a minimum loss in tensile strength.

According to the invention, the graphite fibers are heated at temperatures of about 400 to about l,000 C. in a controlled, atmosphere. The heating means may be by any conventional source of heat including: combustion of hydrocarbons or oxidizable gases or liquids; radiant heating, e.g. from electrically heated resistance wires or bars; inductive heating, e.g. by

direct coupling or radiofrequency energy to the heated work object or indirectly by radiation from a susceptor; microwave or dielectric heating; are plasma; laser or maser heating; etc. We have found, for the purposes of this invention, that the preferred method of heating graphite fibers and yarn is by passing an electric current through the material thereby providing heat from the natural resistance of the material to the flow of electricity. This method of heating is preferred not only for its convenience, speed and efficiency but also because it yields a superior product by its reproducibility and ease of control.

Coating compounds found useful in this invention are soluble metal hydroxides, e.g. sodium, potassium, barium or calcium hydroxide; peroxides, e.g. sodium or potassium peroxide; halides, e.g. sodium, magnesium or nickel chloride; vanadium oxychloride or chloroplatinic acid; potassium or cadmium bromide; iron (II) or zinc iodide; nitrates, e.g. sodium, calcium, vandium, silver or palladium nitrate; nitrites, e.g. sodium or potassium nitrite; permanganates, e.g. potassium permanganate; dichromates, e.g. sodium or potassium dichromate; and sulfides, e.g. sodium or potassium sulfide. By soluble" is meant that the coating compounds are soluble in a liquid solvent such as water, acetone, etc. Although the representative compounds set forth above are soluble in water, it is not essential for the invention that water be used as a solvent in preparing the coating bath. Thus, sodium hydroxide may be applied from an ethyl alcohol solution; chloroplatinic acid from a diethyl ether solution; silver nitrate from an acetonitrile solution; potassium permanganate from an acetone solution; etc. The bath concentrate is not critical and may vary widely, e.g. 0.01-l0 percent by weight, as long as an effective amount of coating is deposited on the fibers prior to heat treatment. If the wet pickup of a 0.5 percent solution on the fibers is percent, the dry addon of coating is 0.5 percent. The temperature of the bath is not critical, and it may be above ambient for increasing the solubility of compounds which are only slightly soluble at ambient conditions. Mixtures of compounds may be employed provided their solutions are compatible.

Although the coating compounds which are effective in this invention are generally reactive toward graphite at the heattreating conditions employed, it is not known by exactly what mechanism the shear strength is improved. Presumably the surface of the fibers is modified, e.g. by mild oxidation of the graphite, or by formation of a thin film which might possible be a carbide, so that better bonding of fibers to the matrix resin results.

The controlled atmosphere may consist of a nonoxidizing gas such as nitrogen, argon, hydrogen, or helium. The oxygen content is preferably nil but may range up to 20 ppm. At higher oxygen content there may occur a decrease in the fiber strength and in the tensile strength of the composites, as well as pitting which adversely affects the fiber strength.

The dwell time is determined by the improvement in bonding which is desired. As the temperature of heat treatment is increased, the dwell time is decreased. Generally shorter dwell times are required as the percentage addon of the soluble compound coating is increased. Within the range of 400-l, 000 C. and 0.01 to 10 percent of coating, we have found that the preferred dwell time is generally within the range of about l to 60 seconds. The dwell time for obtaining shear strength improvement is readily determined by experimentation as hereinafter set forth. Furthennore, the heat-treating conditions, as to coating addon, fiber temperature and dwell time, are preferably optimized in view of both shear and tensile strength.

For purposes of process control, e.g. in determining the dwell time, the improvement in bonding between the graphite fiber surface and a matrix resin is determined by an interlaminar shear test of a composite. The treated graphite fibers in yarn form are formed into a filament-wound composite with a suitable resin such as an epoxy. The composite is cured, and testing specimens are cut to a suitable size and shape, e.g. 0.08 in. X 0.13 in. X 0.52 in. Shear strength is determined by the short beam shear test ASTM D 2,344-67-1 (modified). The span/depth can be set at 3/l 4/1 or 6/l but is preferably 4/1. A discussion of the test and the effect of span/depth is published by Steingiser, Samuel and Cass. Robert A., Graphite Fiber Reinforced Composites," AFMLTR68 357, Part 1, Nov. 1968, pages 84, 96-107.

The graphite yam-resin composites are conveniently prepared by impregnating and winding the yarn into a glass mold. This consists basically of three pieces of glass, an inner rectangular fonn and two outer end pieces, held together by clamps. As the assembly is rotated in its plane, the filament winding is confined between the end pieces and assumes the thickness and shape of the inner form, e.g. 0.070 in. in thickness. A width of about 0.10 in. is obtained by winding 70 turns of yarn. After the composite is wound, it is freed of excess resin and cured.

The heut-trcuted fibers of this invention are useful for reinforcing rcsin matrix composites. Typical resins used in conjunction with graphite fibers are epoxies, polyesters, polyimides, phenolics, and furane resins. The composites are highstrength structural materials useful in aircraft components.

BRIEF DESCRIPTION OF THE DRAWING Some of the novel features of the present invention will become apparent from the following description which is to be considered in connection with the accompanying drawing.

The drawing is a representation of a continuous process for treating graphite yarn. The graphite yarn 1 is unwound from supply reel 2 which is turned by an unreeling motor controlled by dancer" 3. The dancer consists of a pulley which floats on the graphite yarn and operates a microswitch. As downstream tension develops on the yarn, the dancer moves upward and starts the unreeling motor to supply more yarn and thereby maintain a predetermined tension, e.g. 10-100 grams. The yarn passes over pulleys 4, 5, 6, 8, 9, l0, 11, 12, and 16, thence to drum 17. The yarn is heated in three separate states: (A) between electroconducting pulleys 4 and 5 by an electric current to remove sizing and foreign matter; (8) between electroconducting pulleys 8 and 9 to remove solvent and to dry the yarn; and (C) between electroconducting pulleys 4 and 5 by an electric current to remove sizing and foreign matter; (B) electroconducting pulleys 8 and 9 to remove solvent and to dry the yarn; and (C) between electroconducting pulleys l1 and 12 to obtain the necessary heat-treating temperature in the controlled atmosphere Between stages A and B the yarn passes over pulley 6 in pan 7 containing a solution of the soluble compound, e.g. sodium peroxide in water. Pulleys I1 and 12 are contained with chamber 13 provided with ports 14 and 15 for entry and exit of the controlled atmosphere.

DESCRIPTION OF THE PREFERRED EMBODlMENTS The invention is further illustrated by, but not limited to, the following examples.

EXAMPLE This example illustrates the continuous treatment of graphite yarn, in which, first, the sizing was removed by a heat treatment, then a coating of solid oxidizer was applied, and finally the coated fibers were heated to modify the graphite surface.

Commercially available yarn was used, having about L440 filaments per strand, e.g. Union Carbide Corporations Thornel" 50 graphite yam as described in their Technical Information Bulletin No. 465-203-GG.

Referring to the drawing, the yarn 1 passed over pulleys 3, 4, 5, 6. 8. 9, 10, ll, 12, and 16. Between electroconducting pulleys 4 and 5 the yarn was heated to a temperature of about 400 C. for about 30 seconds in air to remove sizing and various impurities. At pulley 6 the yarn entered a bath contained in pan 7. The bath consisted of a solution of a soluble coating comgounds under test, e.g. sodium peroxide, silver nitrate, etc. etween electroconductmg pulleys 8 and 9 the yarn was heated to about 150 C. for about 30 seconds to remove solvent, e,g., water, and thereby form a coating on the fibers. Between electroconducting pulleys 11 and 12 the coated yarn was heated to the desired heat-treating temperature, e.g. 600, 700 C., etc. in an atmosphere of prepurified nitrogen for about 35 seconds. An enclosure 13 was provided for maintaining a controlled atmosphere around the yarn.

The treated yarn was evaluated in composites prepared from a resin made up of parts by weight of (a) a mixture of a cycloaliphatic epoxide and a bisphenol-A based epoxide, e.g. Union Carbidess ERL-2,256 described in their Product Standards dated Nov. l, l964 and 27 parts by weight of (B) adiamine hardener, specifically a eutectic of methylene dianiline and m-phenylenediamine, e.g. Union Carbides ZZL-0820. Each composite was about 0.08 in. thick by 0.10 in. wide. Each was cured 2 hours at 80 C., and 4 hours at l50 C. The shear strengths of the composites were determined by the A.S.T.M. Proposed Tentative Method of Test for Apparent Horizontal Shear Strengths of Flat Laminates," Designation D-2,344-67-T (revised). Specimens 0.08 in. X 0.13 in. X 0.52 in. in size were used at a span-to-depth ration of 4/1. The results from four specimens were averaged.

It is clear from the data in the table that the shear strength of the composites was improved by treatment of the graphite fibers with the coating compound and subsequently heating at above 400 C. 7

HEAT -TREATED GRAPHITE FIBERS Bath Coating Concn. Reaction Compound Weight 8 Temp. *0. M3 0 0.056 400 0.56 400 5.6 400 10.0 650 0.056 720 0.56 720 5.6 720 56.0 720 10.0 750 10.0 850 5.6 900 56.0 900 NaNO; 0.85 600 0.85 650 0.85 700 0.85 750 0.85 800 Ca(NO 0.07 700 0.07 800 0.16 700 0.16 800 1.03 600 1.03 700 1.03 800 Fiber Properties Composite Properties Tensile Weight Av. Shear Improvement Improvement Strength Change Strength Over Heated Over Unheated QrS-i. 1.5.1. Control 2 Control 2 AgNO; 1 .64 720 1.64 000 14.5 600 Pd(-NO z o. 22 600 .22 700 0. 22 000 NaNO 0.65 600 0.65 700 0.65 800 Na Cr 0 0. 021 720 0. 23 720 2 s 720 23. 0 720 0.023 1000 0.23 1000 2.3 1000 Ca (0H) 2 0. 07 600 .07 700 0.07 800 NaCl 0. 050 720 0.56 720 0. 050 000 0.50 800 0.058 900 0.58 900 voc1 0. 007 000 0.007 900 H PtCl 0. 04 720 0.04 000 0.04 900 NaNO; 0.65 700 0.65 750 0.65 000 What we claim is: l. A process of treating a high-modulus graphite fiber to imb. heating the coated fiber in a substantially inert atmosphere containing not more than 20 p.p.m. of oxygen at a temperature of from about 400 to about 1.000 C. for a time sufi'lcient to improve the shear strength of a graphite fiber-resin matrix composite over the shear 299 0.0 5700 0 16 Z86 +1.2 5700 0 16 257 +0.1 7600 27 SS 205 #0 2 7500 53 216 0.0 7900 21 61 186 -0.8 7700 19 57 273 -2 5 6200 B 26 273 -0 8 5800 1 18 Z14 -2.0 8600 43 75 101 1 .2 6800 13 39 177 -1.5 7700 18 S7 149 2 4 7700 18 57 strength of a control composite made with untreated graphite fibers.

2. A process of claim 1 in which the soluble coating com- 5 5 pound is an alkali or alkaline earth metal compound.

3. A process of claim 1 in which the soluble coating compound is an alkali or alkaline earth metal hydroxide, peroxide, halide, nitrate, nitrite, permanganate, dichromate, or sulfide.

4. A process of claim 1 in which the atmosphere is substan- 0 tially nitrogen.

5. A process of claim 1 in which the time is in the range of about I to seconds.

6. A process of claim 1 in which the fiber is in the form of a yarn and is moved continuously through the treating process. 

2. A process of Claim 1 in which the soluble coating compound is an alkali or alkaline earth metal compound.
 3. A process of claim 1 in which the soluble coating compound is an alkali or alkaline earth metal hydroxide, peroxide, halide, nitrate, nitrite, permanganate, dichromate, or sulfide.
 4. A process of claim 1 in which the atmosphere is substantially nitrogen.
 5. A process of claim 1 in which the time is in the range of about 1 to 60 seconds.
 6. A process of claim 1 in which the fiber is in the form of a yarn and is moved continuously through the treating process. 